Nowadays, attempts have been made to reduce exhaust gases such as CO2 from the viewpoint of global environment conservation: In the automobile industry, consideration is given to taking measures to reduce the amount of exhaust gases by increasing fuel efficiency through the weight reduction of an automobile body.
Examples of a method for reducing the weight of an automobile body include a method in which the thickness of a steel sheet which is used for an automobile is decreased by increasing the strength of the steel sheet. It is known that there is a problem with this method in that bending workability decreases with an increase in the strength of a steel sheet. Therefore, there is a demand for a steel sheet having a high strength and good bending workability at the same time.
There is a tendency for a variation in the mechanical properties of a product to increase with an increase in the strength level of a high-strength steel sheet, and there is an increase in variation in bending workability within a product in the case where a variation in mechanical properties is large. It is important that a variation in bending workability within a product does not become large, and, for example, there is a demand for stability of bending workability throughout a product from the viewpoint of increasing the yield of parts in the case where a part is manufactured by performing form molding which involves many portions to be subjected to bending work. Here, the term “a product” refers to a high-strength steel sheet. Therefore, the term “a variation in mechanical properties within a product” refers to a case where, when bending workability is determined at various positions, there is a variation in the determined result. In addition, a variation in properties in the width direction of a steel sheet, which is a product, is regarded as a problem.
In response to such a demand, for example, Patent Literature 1 discloses a high-proportion-limit steel sheet excellent in terms of bending workability and a method for manufacturing the steel sheet. Specifically, Patent Literature 1 discloses a method in which a proportion limit and bending workability are increased at the same time by performing cold rolling on a steel sheet having a specified chemical composition and by then annealing the cold-rolled steel sheet in a specified range of the temperature which is equal to or lower than the recrystallization temperature in order to allow the rearrangement of dislocations to occur while inhibiting excessive recovery. In Patent Literature 1, bending workability is evaluated by performing a 90-degree V-bending test. However, since no consideration is given to the position to be evaluated in Patent Literature 1, it can be said that the stability of bending workability is not improved by the method in Patent Literature 1. Moreover, in the case of the method according to Patent Literature 1, since long-time annealing in a batch annealing furnace is indispensable after cold rolling has been performed, there is a problem of a decrease in productivity in comparison with continuous annealing.
Patent Literature 2 discloses a steel sheet excellent in terms of bending workability and drilling resistance. Specifically, Patent Literature 2 discloses a method in which bending workability is increased, for example, by rapidly cooling a steel sheet after rolling has been performed or after rolling followed by reheating has been performed in order to form a microstructure including mainly martensite or a mixed microstructure including martensite and lower bainite and by controlling the value of Mn/C to be constant over the full range of the C content disclosed. In patent Literature 2, bending workability is evaluated by using a press bending method. However, since no consideration is given to the position to be evaluated in Patent Literature 2, it can be said that stable bending workability is not increased by the method in Patent Literature 2. Moreover, in Patent Literature 2, although specification regarding Brinell hardness is defined, specification regarding tensile strength is not disclosed.
Patent Literature 3 discloses a high-strength steel sheet excellent in terms of bendability and a method for manufacturing the steel sheet. Specifically, Patent Literature 3 discloses a method in which a steel sheet having good close-contact bending capability in all of the rolling direction, the width direction, and the 45-degree direction is manufactured by heating steel having a specified chemical composition, by then performing rough rolling, by performing hot finish rolling which is started at a temperature of 1050° C. or lower and finished in a temperature range from the Ar3 transformation temperature to (the Ar3 transformation temperature+100° C.), by then cooling the hot-rolled steel sheet at a cooling rate of 20° C./s or less, by then coiling the cooled steel sheet at a temperature of 600° C. or higher, by then performing pickling, by then performing cold rolling with a rolling reduction of 50% to 70%, by then performing annealing for 30 seconds to 90 seconds in the temperature range in which an (α+γ)-dual phase is formed, and by then cooling the annealed steel sheet to a temperature of 550° C. at a cooling rate of 5° C./s or more. In Patent Literature 3, bending workability is evaluated by performing close-contact bending. However, since no consideration is given to the position to be evaluated in Patent Literature 3, it can be said that stability of bending workability is not improved by the method in Patent Literature 3. In addition, in Patent Literature 3, since tensile property is evaluated by performing a tensile test and the steel sheet has a strength of less than 980 MPa, it cannot be said that the steel sheet has a sufficient strength for a high-strength steel sheet to be used for an automobile.